Recently, pyroelectric type infrared sensors have been used for measuring temperatures of foods cooking in microwave ovens, controlling room temperature using air conditioners, detecting motion in automatic doors or alarm devices, and the like because the pyroelectric type infrared sensors can detect temperature or other elements without contact. The field of application for these devices is continually expanding.
A pyroelectric type infrared sensor uses the pyroelectric effect of a lithium tantalate (LiTaO.sub.3) single crystal and the like. A pyroelectric substance has a self polarization characteristic, so surface charges are always generated, but the surface charges combine with the charges in the atmosphere to maintain a electrically neutral state when in a stationary condition. Once infrared rays incident the pyroelectric substance, the temperature of the pyroelectric substance is changed, accordingly the neutral charge state on the surface is destroyed and changed. When this occurs, the charge generated on the surface is detected to measure the amount of the incident infrared rays. Thus, a pyroelectric type infrared sensor is created. Generally, pyroelectric substances emit infrared rays according to their temperatures. Thus, the existence or the temperature of objects can be detected by using the pyroelectric type infrared sensor.
A conventional pyroelectric type infrared sensor is structured as shown in FIG. 20. A pyroelectric element 21 is formed of ceramics and detects infrared rays. A sealed tube 22 covers the pyroelectric element 21 to protect it from disturbance light and electromagnetic noise. An incident infrared ray filter 24 is installed at the opening 23 of the sealed tube 22. An external lens 25 is placed outside of the sealed tube 22 and focuses or images the infrared rays 26 emitted from the object onto the pyroelectric element 21. An external lens 25 is a refraction type fresnel lens formed of polyethylene resin utilizing the refraction action of light. The depth T of each of the recesses or slots of the fresnel lens is increased as the position of the recesses or slot moves outwardly so that the declining angle of the slot becomes larger, the light is refracted at the oblique surface of the recsses or slot and focused. The recesses or slots are arranged in equal distances, and the distances between the slots and the depths of the slots are from hundreds to thousands times larger than the wavelength. The dimension of the slots is also large in comparison to the wavelength.
In this conventional pyroelectric type infrared sensor, the external lens 25 is a refraction type lens disposed outside of the sealed tube 22 and focuses or images the infrared rays 26 emitted from the object onto the pyroelectric element. The size of the external lens 25 is large due to the structure of the conventional pyroelectric infrared sensor as described above. Further the size of pyroelectric sensor is also large due to the positional relationship between the external lens 25 and the sealed tube 22. Thus, it is difficult to create a compact pyroelectric sensor.
Another problem with the conventional pyroelectric sensor is that the infrared rays 26, focused and imaged onto the pyroelectric element 21, have to pass the external lens 25. Accordingly the quantity of the light incident on the pyroelectric element 21 becomes very small due to the influence of the reflection and absorption of the light when light passes the external lens. This effect results in lowering of the detection sensitivity.
The conventional pyroelectric type infrared sensor does not provide the important advantage of compact size and improved detection of infrared rays.